Spiral antennas: types and photos

2024 Author: Leah Sherlock | [email protected]. Last modified: 2023-12-17 05:25

Spiral antenna belongs to the class of traveling wave antennas. Its main operating range is decimeter and centimeter. It belongs to the class of surface antennas. Its main element is a spiral connected to a coaxial line. The spiral creates a radiation pattern in the form of two lobes emitted along its axis in different directions.

Spiral antennas are cylindrical, flat and conical. If the required operating range width is 50% or less, then a cylindrical helix is used in the antenna. The conical helix doubles the reception range compared to the cylindrical one. And flat ones already give a twenty-fold advantage. The most popular for reception in the VHF frequency range was a cylindrical radio antenna with circular polarization and a high output signal gain.

Antenna device

The main part of the antenna is a coiled conductor. Here, as a rule, copper, brass or steel wire is used. A feeder is connected to it. It is designed to transmit a signal from the helix to the network (receiver) and vice versa (transmitter). Feeders are of open and closed type. Open type feeders areunshielded waveguides. A closed type have a special shield against interference, which makes the electromagnetic field protected from external influences. Depending on the frequency of the signal, the following design of feeders is determined:

- up to 3 MHz: shielded and unshielded wired networks;

- 3 MHz to 3 GHz: coaxial wires;

- 3GHz to 300GHz: metal and dielectric waveguides;

- above 300 GHz: quasi-optical lines.

Another element of the antenna was the reflector. Its purpose is to focus the signal onto the helix. It is made mainly from aluminium. The basis for the antenna is a frame with a small dielectric constant, for example, foam or plastic.

Calculation of the main dimensions of the antenna

Calculation of a spiral antenna begins with determining the main dimensions of the helix. They are:

- number of turns n;

- turn angle a;

- spiral diameter D;

- pitch of the spiral S;

- reflector diameter 2D.

The first thing to understand when designing a helical antenna is that it is a resonator (amplifier) of the wave. Its feature was the high input impedance.

The type of waves excited in it depends on the geometric dimensions of the amplification circuit. Neighboring turns of the spiral have a very strong influence on the nature of the radiation. Optimal ratios:

D=λ/π, where λ is the wavelength, π=3, 14

S=0, 25 λ

a=12˚

Becauseλ is a value that varies and depends on frequency, then the average values of this indicator calculated by the formulas are taken in the calculations:

λ min=c/f max; λ max=c/f min, where c=3×108 m/sec. (speed of light) and f max, f min - the maximum and minimum parameter of the signal frequency.

λ cf=1/2(λ min+ λ max)

n=L/S, where L is the total length of the antenna, determined by the formula:

L=(61˚/Ω)2 λ cf, where Ω is the polarization-dependent directivity of the antenna (taken from reference books).

Classification by operating range

According to the main frequency range, transceivers are:

1. Narrowband. Beam width and input impedance are highly frequency dependent. This suggests that the antenna can operate without retuning only in a narrow wavelength spectrum, approximately 10% of the relative bandwidth.

2. Wide range. Such antennas can operate over a wide frequency spectrum. But their main parameters (SOI, radiation pattern, etc.) still depend on the change in wavelength, but not as much as narrowband ones.

3. Frequency independent. It is believed that here the main parameters do not change when the frequency changes. These antennas have an active region. It has the ability to move along the antenna without changing its geometric dimensions, depending on the change in wavelength.

The most common are helical antennas of the second and third types. The first type is used whenincreased "clarity" of the signal at a certain frequency is needed.

Self-made antenna

The industry offers a wide variety of antennas. A variety of prices can vary from a few hundred to several thousand rubles. There are antennas for television, satellite reception, telephony. But you can make a spiral antenna with your own hands. It's not that hard. Helical Wi-Fi antennas are especially popular.

They are especially relevant when it is necessary to amplify the signal from the router in some large house. To do this, you need a copper wire with a cross section of 2-3 mm 2 and a length of 120 cm. It is necessary to make 6 turns with a diameter of 45 mm. To do this, you can use a tube of the appropriate size. A shovel handle fits well (it has about the same diameter). We wind the wire and get a spiral with six turns. We bend the remaining end in such a way that it passes exactly through the axis of the spiral, “repeating” it. We stretch the screw part so that the distance between the turns is within 28-30 mm. Then we proceed to the manufacture of the reflector.

For this, a piece of aluminum 15 × 15 cm in size and 1.5 mm thick will do. From this blank we make a circle with a diameter of 120 mm, cutting off unnecessary edges. Drill a 2mm hole in the center of the circle. We insert the end of the spiral into it and solder both parts to each other. The antenna is ready. Now you need to remove the radiation wire from the antenna module of the router. And solder the end of the wire withend of the antenna coming out of the reflector.

433 MHz antenna features

First of all, it must be said that radio waves with a frequency of 433 MHz during their propagation are well absorbed by the ground and various obstacles. For its retransmission, low power transmitters are used. As a rule, various security devices use this frequency. It is specially used in Russia, in order not to interfere on the air. The 433 MHz helical antenna requires a higher output gain.

Another feature of using such transceiver equipment is that the waves of this range have the ability to add the phases of the direct and reflected waves from the surface. This can either increase the signal strength or weaken it. From the above, we can conclude that the choice of the "best" reception depends on the individual setting of the antenna position.

Homemade 433 MHz antenna

It is easy to make a 433 MHz helical antenna with your own hands. She is very compact. To do this, you need a small piece of copper, brass or steel wire. You can also use just wire. The wire diameter should be 1 mm. We wind 17 turns on a mandrel with a diameter of 5 mm. We stretch the helix so that its length is 30 mm. With these dimensions, we test the antenna for signal reception. By changing the distance between the turns, by stretching and compressing the helix, we achieve a better signal quality. But you need to know that such an antenna is very sensitive to various objects,brought close to her.

UHF receiving antenna

UHF helical antennas are necessary for receiving a television signal. By their design, they consist of two parts: a reflector and a spiral.

It is better to use copper for the helix - it has less resistance and, therefore, less signal loss. Formulas for its calculation:

- total length of the spiral L=30000/f, where f- signal frequency (MHz);

- helix pitch S=0.24 L;

- coil diameter D=0, 31/L;

- spiral wire diameter d ≈ 0.01L;

- reflector diameter 0.8 nS, where n- number of turns;

- distance to the screen H=0, 2 L.

Gain:

K=10×lg(15(1/L)2nS/L)

The reflector cup is made of aluminum.

Other types of transceiver equipment

Conical and flat helical antennas are less common. This is due to the difficulty of their manufacture, although they have the best characteristics in terms of signal transmission and reception. The radiation of such transmitters is formed not by all the turns, but only by those whose length is close to the wavelength.

In a flat antenna, the helical line is made in the form of a two-wire line twisted into a spiral. In this case, adjacent turns are excited in phase in the traveling wave mode. This leads to the fact that a radiation field with circular polarization is created towards the axis of the antenna, allowing you to create a wide frequency band. There are flat antennas with the so-called spiralArchimedes. This complex shape allows a significant increase in the transmission frequency range from 0.8 to 21 GHz.

Comparison of helical and highly directional antennas

The main difference between a helix and a directional antenna is that it is smaller. This makes it lighter, which allows installation with less physical effort. Its disadvantage is a narrower range of receive and transmit frequencies. It also has a narrower radiation pattern, which requires "search" for the best position in space for satisfactory reception. Its undoubted advantage is the simplicity of design. A big plus is the ability to tune the antenna by changing the pitch of the coil and the total length of the spiral.

Short Antenna

For better resonance in the antenna, it is necessary that the "elongated" length of the helical part is as close as possible to the wavelength value. But it should not be less than ¼ wavelength (λ). Thus, λ can reach up to 11 m. This is true for the HF band. In this case, the antenna will be too long, which is unacceptable. One way to increase the length of the conductor is to install an extension coil at the base of the receiver. Another option is to feed the tuner path into the circuit. Its task is to match the output signal of the radio station transmitter with the antenna at all operating frequencies. Speaking in plain language, the tuner acts as an amplifier for the incoming signal from the receiver. This scheme is used in car antennas, where the size of the element that receives the radio wave is very important.

Conclusion

Spiral antennas have become very popular in many areas of electronic communications. Thanks to them, cellular communication is carried out. They are also used in television and even in deep space radio communications. One of the promising developments to reduce the size of the antenna was the use of a cone reflector, which makes it possible to increase the length of the receiving wavelength compared to a conventional reflector. However, there is also a drawback, expressed in a decrease in the operating frequency spectrum. Also an interesting example is the "two-way" conical helical antenna, which allows you to work in a wide frequency spectrum, due to the formation of an isotropic directional diaphragm. This is because the power line in the form of a two-wire cable provides a smooth change in impedance.